It caps a memorable twelve months for the 24 year old who, at this point last year, was preparing to link up with his new team mates at Gloucester Rugby for preseason training.
The centre went on to start 21 games in total for the Cherry and Whites last season, including 18 of the club's Aviva Premiership starts.
He earned a call up to the England training squad for the Autumn international series and won his first cap in the RBS Six Nations fixture against Scotland when he scored a try on debut.
He also scored for his country last Saturday against Argentina and was preparing for the second test against the Pumas when the call up came.
Upon hearing the news, Davies expressed genuine delight for Twelvetrees.
"I'm really pleased for Billy. It's a massive achievement. Selection for the Lions is something every player strives for and, speaking on behalf of everyone at Gloucester Rugby, we're all very proud of him.
"Since his arrival at the club, he's been the model professional. He trains hard, he works hard and he's formed a great midfield partnership with Freddie Burns.
"He's almost a blueprint for what you want from a 12 in the modern game. He's got great distribution skills, he's massively physical and he's got a superb kicking game.
"And I honestly think that he's still improving. He's a very intelligent player, who is always looking for ways to improve his game and his selection for the Lions is credit to both himself and the club."